Mr O'Gorman was heavily involved in the Yes campaign that won the referendum on marriage equality in the Republic of Ireland in May.

Following that vote, Northern Ireland is now set to be the only part of the UK or Ireland where civil marriage is denied to same-sex couples.

The Stormont Assembly has rejected a proposal calling for the introduction of gay marriage on four occasions, with unionists opposed to the move using a contentious voting mechanism to effectively veto it.

A number of Christian organisations are opposed to any legal redefinition of marriage, insisting the institution should be restricted to heterosexual couples.

Mr O'Gorman said campaigners in favour of legislative change in Northern Ireland could learn lessons from the Yes campaign south of the border.

"The decision by the Irish people to back equality echoed around the world," he said.

"Nowhere more so than across the border in Northern Ireland, where same-sex couples now find themselves in the only part of the UK or Ireland banned by law from getting married.

"The Yes campaign was won by focusing on a positive message that the lives of LGBTI people, their relationships and their families matter. That they are entitled to the same human rights as others, and to the full and equal protection of the law. It was essentially a family values campaign, which asserted that all families matter, and that none should be excluded from the rights, privileges, protections and responsibilities granted by civil marriage.

"The tone and focus of the campaign was overwhelmingly affirming, positive and even joyous."

Mr O'Gorman claimed polling data showed almost 70% of people in Northern Ireland were in favour of allowing same-sex couples to marry, with the percentage even higher among younger people.

"Politicians can't simply ignore figures like that," he added. "The tide of public opinion has shifted decisively and there can only be one outcome.

"It is now only a matter of time before civil marriage rights for same-sex couples are introduced in Northern Ireland. Amnesty's commitment here and around the world is to ensure that equality for all arrives sooner rather than later. By focusing on an affirmative message of how equality would positively affect the lives of ordinary people across Northern Ireland, many of the remaining doubters can be won over."

The Amnesty International Belfast Pride lecture - Marriage Equality: How the YES was won - will take place in the Clayton Hotel, Ormeau Avenue, at 6.30pm.